196 COMMERCIAL ELECTRICAL TESTING 



Heat Run 



On a new design of transformer a heat run should be 

 made at normal primary current with the secondary 

 short circuited until constant temperature is reached. 

 Temperature rise by resistance should be taken hourly 

 on one transformer out of every five, and thermometer 

 temperatures on each one. When constant tempera- 

 tures are reached, the transformer should run for two 

 hours at the thirty minute load, which will be found 

 stamped on the name plate. On transformers that 

 are duplicates of some that have been previously built, 

 a two hour heat run at the thirty minute load is sufficient. 

 Measure hot resistances on one out of five and take ther- 

 mometer temperatures on all. They should then be 

 run for one minute at the thirty minute load with the 

 secondary open to test the insulation between turns and 

 between layers. This corresponds to the double poten- 

 tial test on other transformers. 



The high potential test may be taken on several 

 transformers at the same time. See that all secondaries 

 and cases are properly connected together. Series trans- 

 formers used in connection with potential regulators have 

 the same characteristics as constant potential trans- 

 formers and are tested as such. 



